1. Field of the Invention
The present invention relates to an organic electroluminescent display, and more particularly, to an organic electroluminescent display with an improved electrode structure.
2. Description of the Related Art
Generally, organic electroluminescent displays are self-luminescent displays which emit light by electrically exciting a fluorescent organic compound. They can be operated with low voltage and can be made thin. In addition, organic electroluminescent displays have advantages, such as a wide viewing angle and a fast response speed, that overcome the problems of liquid crystal displays. Accordingly, they have been noted as next generation displays.
In such organic electroluminescent displays, an organic layer is formed in a predetermined pattern on a substrate made of glass or other transparent materials, and electrode layers are disposed under and below the organic layers.
In organic electroluminescent displays having the above-described structure, when positive and negative voltages are applied to the electrodes, holes are moved from an electrode, to which a positive voltage is applied, to a luminescent layer via a hole transport layer. Electrons are moved from an electrode, to which a negative voltage is applied, to the luminescent layer via an electron transport layer. The electrons meet the holes in the luminescent layer, thereby generating excitons. The excitons make transitions from an excitation state to a ground state, thereby provoking fluorescent molecules of the luminescent layer to emit light. As a result, an image is formed.
The light efficiency of organic electroluminescent displays operating as described above is divided into internal efficiency and external efficiency. The internal efficiency depends on the photoelectric conversion efficiency of an organic luminescent material. The external efficiency is referred to as light coupling efficiency and depends on the refractivity of each of layers included in an organic electroluminescent display. Organic electroluminescent displays have lower external efficiency than other displays such as cathode-ray tubes and plasma display panels (PDPs). Accordingly, such organic electroluminescent displays need to be improved in terms of characteristics of displays, such as brightness and a life span.
In the meantime, in organic electroluminescent displays having the above-described structure, as the size of a panel increases, the line resistance of an electrode increases, which causes the brightness of an image to be nonuniform. In particular, when an organic electroluminescent display is driven by a thin-film transistor, the line resistance of a cathode increases, and thus the above problem occurs.
Japanese Patent Publication Nos. sho 62-172691, sho 63-172691, hei 1-220394, and hei 11-283751 disclose conventional organic electroluminescent displays. These conventional organic electroluminescent displays are not provided with an element for reducing the line resistance of a cathode and thus still have a problem of nonuniform brightness of an image due to the line resistance.